In wireless communication systems, a technique used for improving system throughput is the hybrid automatic repeat request (HARQ) technique.
The HARQ technique resends data packets on the detection of errors on the received packets. The HARQ process is activated when the transmission of a new data packet is launched. In a typical implementation of a HARQ process, each packet to be transmitted is first attached with a cyclic redundancy check (CRC) for the purpose of error detection. At the receiver, the contents of each packet are validated through the use of CRC code. If the packet cannot pass the CRC validation, the receiver feeds back a non-acknowledgment (NACK) signal to the transmitter to request a retransmission. At the receiver, the received retransmitted packets and the received original packets are further combined to improve the system throughput. If the received packet is decoded and passes the CRC validation, an acknowledgement (ACK) signal will be sent to the transmitter to acknowledge the successful decoding of the packets and request the transmission of a new data packet. The HARQ process might comprise more than one transmission. The HARQ process will be terminated if the transmitter receives an ACK signal for the transmitted data packet or a predefined maximum number of retransmissions (e.g. 4 retransmissions) is reached.
Two well-known protocols to implement a HARQ process are Chase combining protocol and Incremental Redundancy (IR) protocol.
In Chase combining protocol, the same data packet is retransmitted on a repeat request.
In IR protocol, the originally transmitted packets include system information and some parity information. On the detection of decoding errors, parity information different from that contained in the original packet will be retransmitted to provide more redundancy to assist the decoding of the system information. In contrast to the Chase packets transmitted by the Chase combining protocol, the retransmitted IR packets are not the repetition of the originally transmitted packets.
The capacity of wireless communication systems can be further enhanced by simultaneously transmitting multiple data streams over a multiple input multiple output (MIMO) communication system that employs multiple (NT) transmit antennas and multiple (NR) receive antennas through the use of spatial multiplexing (SM). A well-known technique to increase the capacity through SM is discussed in prior art reference V-BLAST: an architecture for realising very high data rates over the rich-scattering wireless channel” by P W Wolniansky et al in the published papers of the 1998 URSI International Symposium on Signals, Systems and Electronics, Pisa, Italy, Sep. 29 to Oct. 2, 1998 (hereinafter referred to as VBLAST).
MIMO techniques can be used in conjunction with orthogonal frequency division multiplexing (OFDM) to achieve efficient spectral utilization for transmissions over frequency selective fading channels, by transferring frequency selective fading channels into a set of parallel frequency-flat fading and orthogonal subchannels overlapping each other.
In MIMO communication systems, the data streams transmitted over different transmit antennas normally have various error performance levels since each data stream experiences different degrees of link conditions. It is unlikely that all the data streams experience decoding errors simultaneously, especially if a large number of antennas are employed.
The HARQ technique making use of the antenna diversity could be able to increase the throughout of a MIMO system, by employing antenna dependent HARQ transmission scheme, where independent HARQ processes are used for individual data streams transmitted independently from different antennas. For instance, at the transmitter, each of the multiple data streams is attached with an independent CRC code. At the receiver, each of the decoded data streams goes through independent CRC validation and the receiver acknowledges each data stream by sending an indicative signal (ACK/NACK signal) back to the transmitter. The transmitter then decides from the indicative signals which data streams need to be retransmitted. System throughput is improved because only the transmit data streams receiving NACK signals will be retransmitted, or only the antennas (unreliable antennas) transmitting these particular data streams receiving NACK signals will perform retransmissions, while the transmit antennas transmitting data streams receiving ACK signal (reliable antennas) will transmit new data streams.
Normally, in order to reduce the number of retransmissions, the retransmitted data streams have higher requirement on transmission quality than the newly transmitted data streams. The higher the number of retransmissions required is, the lesser the system throughput will be. It is therefore desirable to reduce the number of retransmissions.
A need therefore exists to provide a method and system for re-transmission in a multiple-input multiple-output (MIMO) communication system that seeks to reduce the number of retransmissions required and improve the system throughput.